1. Field of the Invention
This invention relates to magnetic recording of digital information, and more particularly, to a method and apparatus for reading and writing data on a rotating magnetic disk media.
2. Description of Related Art
Data storage devices using rotating disks coated with a layer of iron oxide, or other materials susceptible to the influence of magnetic fields, are well known and have been used for many years. Such magnetic disk data storage devices are currently widely used in computer systems. Typically, data is read from, and written to, disks in concentric rings (referred to as "tracks") on each surface of a disk. Tracks are typically divided by servo cells. The collection of portions of each track on one surface of a disk which lie between two servo cells is referred to as a "data wedge". Accordingly, each such data wedge comprises a pie-shaped area on the surface of the disk. Furthermore, the length of that portion of each track that lies within a data wedge depends upon the radial location of the track (i.e., the distance of the track from the edge of the disk). The portion of a track within each data wedge is further subdivided into sectors of dam.
In order to read and write data onto the surface of a disk, a magnetic read/write head is placed in close proximity to the surface of the disk. The read/write head is aligned with a track and reads the data from the track to determine which particular sector within a track the read/write head is currently over. Each sector on a track is associated with a unique sector ID. Each sector includes an identification header in which the sector ID of the track is stored. By reading the header, a disk controller determines where the read/write head is on the track (i.e., determines the sector ID). Accordingly, the controller can determine when a sector containing data to be read, or the sector to which data is to be written, is under the read/write head. The Controller associates each sector of data on the disk with a physical address and a logical address such that the data can be reliably recovered. The physical address indicates in which sector the data resides. The logical address is assigned and mapped to the physical address at the time the data is written to allow the controller to manage the read/write operation more efficiently.
Efforts to increase the areal density of data that may be written on the surface of a disk have been made in the past and are continuing to be made. Improved techniques for increasing areal recording density have been an important enabling factor in the trend in this field toward smaller yet higher capacity, disk drives. Areal recording density is generally expressed in terms of bits per square inch (or other unit area). Analytically, areal density is the product of the track density (i.e., the number of tracks per inch, or "TPI") on the surface of a disk, and the bit density (i.e., the number of bits per inch, or "BPI") that can be recorded along a particular track.
As demand grows for increased areal density, the amount of disk area dedicated to overhead functions, such as determining the sector ID of a particular sector, becomes a greater burden. FIG. 1 illustrates one typical format used on a disk of a disk data storage device. As shown in FIG. 1, the following fields are typically provided in a header associated with each sector of data.
Servo cell 10--Servo cells are commonly used to position a read/write head over a particular track on the surface of a magnetic disk, in known fashion.
Phase Lock Oscillator Field 12--The phase lock oscillator field allows a phase locked oscillator within the head/drive assembly (HDA) to be synchronized to the bit rate at which data is to be read and written.
Sync Byte Field 14--The sync byte field 14 contains a bit pattern that allows synchronization to the boundaries between bytes of data to be read or written prior to reading or writing the sector ID.
Sector ID Field 16--The sector ID field 16 contains the sector ID of the sector that contains the header.
Circular Redundancy Checking (CRC) Field 18--The CRC field 18 contains a CRC code which allows errors in the information stored in the phase lock oscillator field 12, the byte sync field 14, and the sector ID field 16 to be identified.
PAD Field 20--The pad field provides a gap between the CRC field and a data phase lock oscillator field 22 which follows.
Data Phase Lock Oscillator Field 22--The data phase lock oscillator field contains a pattern that allows the HDA phase lock oscillator to re-lock onto the bit rate before reading data to ensure that the phase lock oscillator has not drifted too far from the required frequency since the last phase lock oscillator field 12 was read.
Data Sync Byte Field 24--The data sync byte field 24 contains a bit pattern that ensures that the HDA is synchronized to the boundaries between bytes of data to be read or written prior to reading the data field 26.
Data Field 26--The data field 26 stores the information that is read and written to the disk storage device during normal operation.
Error Correction Code Field 28--The error correction code field is verified the data read from the data field to allow error correction to be performed.
Data Pad Field 30--The data pad field provides a gap between the end of the ECC field 28 and the beginning of a next phase lock oscillator field 12. The overhead fields 12, 14, 16, 18, 20, 22, 26, and 28 require a substantial amount of area on the disk surface. Furthermore, some disk data storage devices currently use magneto-resistive (MR) heads. MR heads have a read head which is spaced apart from the write head. The read and write heads are typically mounted on a common radial actuator arm (i.e., an arm on which the read and write head are disposed at the proximal end, positions the read and write head over a track of data by pivoting about a fixed point at the distal end of the arm). Because the read and write head become skewed with respect to one another (i.e., a line drawn from the read to the write head is not parallel to the track which the head is over), the read and the write head cannot both be aligned over a track. Therefore, in order to read the header and immediately write data to the track associated with that header, a second header must precede the data section offset from the track containing the data section. Accordingly, a greater area is required for the header. Thus, the area of the disk which is available for storing data is reduced. Therefore, it would be advantageous to provide a method and apparatus for determining the sector ID of a sector of data on the disk without the need to consume disk area.